The present invention relates to a printer for electro photographically forming an image on a continuous-form recording paper.
There are known imaging devices using an electrophotographing process wherein the surface of a photo conductive drum is exposed to an optical image to form a latent image thereon. A toner is applied for development of the image on the drum. The toner image is then transferred to a recording paper and is fixed by means of a fixing device. One of such imaging devices is the printer which is adapted to print information on a continuous-form recording paper as used in conventional line printers.
The continuous-form recording paper is so-called fan-fold paper with sprocket holes (referred to simply as "continuous form" in later description). There are perforations at certain intervals of the paper to facilitate cutting the paper into separate sheets.
Different processes have been available to fix the toner image to the recording paper (continuous form) in such an electrophotographing process, examples of fixing processes are: thermally fusing the toner to the paper, using a solvent or applying a pressure. Thermal fusing process is widely used as it is considered to be the safest and most reliable way of fixing. Among several different types of such thermal fusing processes, most used is a heat roll system in which the recording paper with an image remaining unset is pressed against a heat roller so that the heat transferred from the heat roller fuses and sets the toner image.
The heat roll system uses a fixing roller pair consisting of the heat roller heated to a high temperature and a backup roller, the two rollers being placed in opposition to each other. The recording paper carrying an unfixed image is clamped between the two rollers so that it is pressed against the heat roller with a certain pressure to fix the toner image on the paper with the heat from the heat roller. The usual arrangement is such that the recording paper is traveled forward with rotation of the fixing roller pair itself.
The electrophotographic printer using continuous form usually employs the heat roll system.
In printers using continuous form, however, printing has to start with a certain space from the perforation of the continuous form (i.e., top end of the page). On the other hand, the electrophotographic printing system is so configured that operating elements such as exposure, developer and transfer units are all placed along the circumference of a photoconductive drum. As a result, those individual units are brought into operation as the drum rotates so that the exposure and transfer processes take place in different positions. In order to make the exposure start position coincident with a certain point from the perforation of the continuous form with the start of printing (i.e., to start printing at a certain space from the perforation), it is necessary to adjust the print start position by moving the photoconductive drum relative to the continuous form before starting printing (start positioning). If the drum is kept in contact with the continuous form (held in a transfer operative position), the photoconductive material coated on the drum may be damaged or worn so as to shorten the life of the drum. This may also cause the continuous form to be soiled with toner remaining on the drum surface.
Another problem encountered is that when using the heat roll system as mentioned above, the heat roll must be constantly kept heated even in a print stand-by state, because heating the heat roller takes a considerable time. As a result, the recording paper (continuous form) constantly pressed between the heat roller and the backup roller is at its a certain section steadily exposed to the heat transferred from the heat roller, resulting in the paper being burnt or blistered.
The assignee of the present application has proposed an arrangement in which one of the fixing roller pair can be withdrawn from the other to avoid contact of the paper with the heat roller in the printing stand-by state and in which the transfer charger can be withdrawn from its transfer operating position to separate the recording paper from the drum. However, fabrication costs become high as separatre driving means have been employed for withdrawing the fixing roller and the transfer charger.
Further, the surface of the continuous form has paper dust particles deposited thereon which are developed during formation of sprocket holes and perforations. Such dust particles are likely to enter the transfer charger or be stuck onto the drum surface, resulting in various problems. To eliminate these problems, a cylindrical dust brush unit is provided near a paper inlet portion of the printer. The dust brush, in contact with the continuous form, is rotated in the direction opposite to the paper feed direction for brushing dust particles off the surface of the paper.
However, when the transfer charger and one of the fixing roller pair are withdrawn from their transfer operative and fixing operative positions during the print stand-by state, the continuous form can no longer retained at the position. As a result, the continuous form moves in opposite direction by the force exerted by brushing with the dust brush so that the print start position is deviated from the position.
Moreover, there has been a problem that the sensitized paper is attracted to the drum surface and can not be separated therefrom merely by withdrawing the transfer charger from its transfer operative position. Thus, an arrangement has been proposed such that a paper holder is provided on a pivotal member carrying the transfer charger so that the continuous form is passed between the paper holder and the transfer charger, whereby the continuous form is separated from the drum surface by means of this paper holder upon withdrawal movement of the transfer charger. With this arrangement, however, there has been a difficulty in loading the continuous form because the form has to be passed between the paper holder and the transfer charger.